PK Shoulder Joint Sample

PILATES KINESIOLOGY

The Shoulder joint

Anatomy and Kinesiology review

Exercise analysis: Hug a tree Pulling straps Shoulder press Discussion: The inherent instability of the glenohumeral joint Self study questions

Image 3, the elevators of the scapula. The trapezius, on the right side. The trapezius has been cut away on the left to reveal the deeper layers.

Image 4. The direct line of force of the muscles responsible for elevation of the scapulothoracic joint are indicated with red arrows. Note that the rhomboids can also function as retractors of the scapula. See discussion below

PROTRACTORS The serratus anterior is the prime protractor at the scapulothoracic joint. This force is employed for forward pushing and reaching activities. Pectoralis minor also assists with this function. See Image five

Image 5

RETRACTORS The middle trapezius has an optimal line of force to retract the scapula. The rhomboids and the lower trapezius muscles function as secondary retractors. All the retractors are particularly active during pulling activities, such as climbing and rowing. These muscles secure the scapula to the axial skeleton. (See Image 6) The secondary retractors provide an example of how muscles can share similar actions, but also function as direct antagonists to one another. During a vigorous retraction effort, the elevation tendency of the rhomboids is neutralized by the depression tendency of the lower trapezius. The line of forces of both muscles combine, however, to produce pure retraction.

Image 6. A posterior view of the middle and lower trapezius rhomboids cooperating to retract the scapulothoracic joint.

The dashed line of force of both the rhomboid and lower trapezius combines to yield a single retraction force, shown by the straight arrow.

Dynamic stability and the rotator interval As previously mentioned, the GH joint capsule receives significant structural reinforcement from the four rotator cuff muscles. (See Image two) These four muscles form a cuff that protects and actively stabilizes the GH joint, especially during dynamic activities like “hug a tree.” Important to note is that the rotator cuff fails to cover two regions of the capsule: inferiorly, and a region between the supraspinatus and subscapularis known as the rotator interval, a common site for dislocation. 3 The inherently weakened region of the rotator interval, however, is reinforced by the tendon of the long head of the biceps and the coracohumeral ligament. 4 Cadaver studies strongly suggest that in an active state, the long head of the biceps restricts anterior translation of the humeral head. 5 In the “Hug a tree” the long head of biceps contracts to produce motion at the GH and to restrict anterior translation of the humeral head.

Image two. The four rotator cuff muscles form a cuff that protects and actively stabilizes the GH joint, as the anterior deltoid horizontally adducts the humerus. The Supraspinatus (1), infraspinatus (2) and teres minor (3) pull the head of the humerus into the glenoid fossa. The Serratus anterior contracts to protract the scapula.

Image 3. The primary protractors of the scapula are the serratus anterior and the pectoralis minor. These muscles form a force-couple to effectively protract the scapula and provide stable attachments for the more distal mobilizer, such as the deltoid and bicpes brachi. The serratus anterior also secures the medial border of the scapula firmly against the thorax by generating an external rotation torque.(See discussion below on external rotation of the scapula).

Hug A tree Osteokinematics Osteokinematics describes the motion of bones relative to the three cardinal (principal) planes of the body: sagittal, frontal, and horizontal. The Hug a tree exercise provides an opportunity to look briefly at the osteokinematics of the AC joint. Image 4 below shows the primary motion of upward and downward rotation of the AC joint. Lessor known “secondary motions� - horizontal and sagittal plane adjustments- are also possible. During protraction of the scapula, as in Hug a Tree, internal rotation occurs. During scapulothoracic elevation, anterior tilting occurs.

Image 4

The Shoulder press- single or double arms

The shoulder press combines motions at the scapulothoracic, sternoclavicular, acromioclavicular, glenohumeral and humeroulnar joints. Consequently, a multitude of muscular interactions are required to abduct the shoulder and extend the elbow to press the carriage away from the resting position. As we have seen the scapulothoracic joint serves as an important mechanical platform for all active movements of the humerus. Muscles such as the deltoid and rotator cuff require coactivation of the serratus anterior and trapezius to effectively stabilize the scapula and clavicle.

In turn, the proximal skeletal attachments of the scapulothoracic muscles on the cranium, ribs and spine must be stabilized so that they can stabilize the scapula and clavicle.

(Note to student: Similar muscular interactions have been covered in this module. As a result, in the

analysis below we will focus a little more on the arthrokinematics of the exercise. ‘Arthrokinematics’ refers to the movement of joint surfaces. The angular movement of bones in the human body occurs as a result of a combination of rolls, spins, and slides.)

Scapulothoracic, acromioclavicular sternoclavicular Joint motion Pressing the carriage away involves full shoulder abduction including about 60 degrees of scapular upward rotation. The upwardly rotated scapula provides a stable yet mobile base for the abducting humeral head and projects the glenoid fossa upward and anterior-laterally. Upward rotation also maximizes the volume within the subacromial space, preventing impingement, and preserves the optimal length-tension relationship of the abductor muscles of the glenohumeral joint, such as the middle deltoid and supraspinatus. See Image 1

Image 4

The primary upward rotator muscles of the scapula The primary upward rotator muscles during the shoulder press are the serratus anterior and the upper and lower fibers of the trapezius. These muscles form a force-couple to effectively upwardly rotate the scapula 7, driving the upward rotation of the scapula and providing stable attachments for the mobilizers, such as the deltoid and rotator cuff muscles. The serratus anterior also secures the medial border of the scapula firmly against the thorax by generating an external rotation torque.

Image 5 The force-couple of the serratus anterior (SA) and trapezius (UT-upper trapezius, LT-lower trapezius) rotates the scapula in the same rotary direction as the abducting humerus.

Image 7 Subscapularis, exerts a downward translational force on the humeral head to counteract excessive s uperior translation.

Arthrokinematics at the Glenohumeral Joint The roll-and-slide arthrokinematics depicted in image8 are essential to the completion of full range abduction.

Because the longitudinal diameter of the articular surface of the humeral head is almost twice the size of the longitudinal diameter on the glenoid fossa, a simultaneous roll and slide are essential to enable allow a larger convex surface to roll over a much smaller concave surface without running out of articular surface. Without a sufficient concurrent inferior slide during abduction, the superior roll of the humeral head would ultimately lead to a jamming of the head against the unyielding coracoacromial arch. This situation would create an impingement of the supraspinatus tendon and subacromial bursa between the head of the humerus and the coracoacromial arch. Such an impingement can physically block further abduction. Without a sufficient concurrent inferior slide during abduction, the superior roll of the humeral head would ultimately lead to a jamming of the head against the unyielding coracoacromial arch. Repeated compression may damage and inflame the supraspinatus tendon, subacromial bursa, long head of the biceps tendon, and superior parts of the capsule. Over time, this repeated compression may lead to a painful condition known as subacromial impingement syndrome.

Image 8